Field of the Invention
The present invention relates to a structure of an optical waveguide element manufactured on a substrate, and in particular, relates to a polarization conversion element which is for converting a polarized wave.
Description of the Related Art
The amount of information used in optical communication has steadily increased recently. Thus, in order to cope with such an increase in the amount of information, a countermeasure such as an acceleration in signal speed and an increase in the number of channels due to wavelength multiplex communication has proceeded in an optical communication network such as backbone, metro, and access. According to this, a system necessary for optical communication becomes complex, and problems such as an increase in the size or an increase in the cost, and an increase in the power consumption of the system occur.
In addition, similarly, even in data centers, which have increased in number in recent years, it is urgently necessary to cope with an increase in the amount of information. In communication between computers in the data center, an electrical signal is mainly transmitted through a metal cable of the related art, but recently, optical communication using an optical fiber has been used from a demand for a further acceleration or a further decrease in the power consumption. Further, even in a board, in a CPU, and each level of a computer, the introduction of optical communication becomes a problem.
As a technology for solving such problems in the optical communication network and for realizing the introduction of optical communication to a new field, recently, an optical device using a high refractive index material such as silicon, InP, and GaAs has attracted attention in addition to an optical integrated circuit of quartz PLC which has been used in the related art, a high-speed operating device of a ferroelectric body such as lithium niobate, and the like, and research and development of a substrate type optical waveguide element device has proceeded in many fields.
The wavelength of light in a medium is in reverse proportion to the refractive index of the medium, and thus in silicon having a high refractive index of approximately 3.5, the dimension of an optical waveguide such as a core width decreases. In addition, a medium having a largely different refractive index with respect to silicon such as silica is set to a cladding, and thus an optical waveguide having strong light-trapping properties is obtained. Regarding characteristics thereof, a radius of curvature can be decreased. For these reasons, it is possible to reduce the size of the optical device using the optical waveguide, a reduction in the size can be realized in the same function, and a plurality of functions can be realized in the same size. In addition, electrical control can be performed by using the fact that silicon is a semiconductor material, and a property variable device such as an optical modulator can be realized (refer to PCT International Publication No. WO00/58776).
Further, a technology and a device relevant to manufacturing of the optical device using silicon have many common elements with a technology and a device relevant to a semiconductor process used in manufacturing of a semiconductor device such as a CPU and a memory of the related art. It is possible to expect that an optical device can be realized with low cost due to mass production. By integrating the semiconductor device and the optical device of the related art on the same substrate, it is possible to connect the semiconductor device to the optical device on the substrate. So far, an electrical signal on metal wiring has been used for the connection between the devices, but in the future, a part of the signal will be replaced with light, and thus a further acceleration in an apparatus and a reduction in power consumption are likely to be realized.
A planar optical waveguide used in such a substrate type optical component has asymmetry in an azimuth direction in a sectional direction of the waveguide, unlike a cylindrical symmetrical optical fiber. Accordingly, the planar optical waveguide has different properties with respect to waveguide light (polarization) in a different deflection direction. In the planar optical waveguide, for the sake of convenience, a waveguide mode in which a main electric field is in a horizontal direction with respect to the substrate is indicated by a TE mode, and a waveguide mode in which a main electric field is in a vertical direction with respect to the substrate is indicated by a TM mode. When the structure of the planar optical waveguide in the vertical direction is different from the structure of the planar optical waveguide in the horizontal direction, the two modes have different effective refractive indices. For this reason, it is difficult to manufacture a device having the same properties with respect to both of the polarizations on the flat substrate. Therefore, a structure referred to as polarization diversity is used in which the two modes are rotated. In this polarization diversity, a polarization rotator which performs conversion of the polarization between the TE mode and the TM mode is necessary.
In addition, in a recent high-speed optical communication method, a polarization multiplexing technology is performed in which different signals are imposed on two polarizations which are orthogonal to each other at the time of transmitting the optical fiber, and in this transceiver, an elemental technology of separating, multiplexing, or converting the polarization is necessary.
Among them, the following elements have been considered as a substrate-integrated polarization conversion element.
In L. Chen, C. R. Doerr, and Y.-K. Chen, “Compact polarization rotator on silicon for polarization-diversified circuits,” Optics letters, Vol. 36, Issue 4, pp. 469-471 (2011), a polarization conversion element manufactured on a silicon substrate is disclosed. In the above-described document, a structure is formed on an upper portion of a waveguide by using silicon nitride (Si3N4) having a different refractive index with respect to the waveguide of silicon. However, a process using Si3N4 is necessary, and thus it is difficult to form the waveguide. In addition, in a portion to which Si3N4 is applied, ideally, it is preferable that a tip end of Si3N4 be manufactured to be extremely narrow, but in an actual process for mass production, the limit is approximately 100 nm, and thus a loss occurs due to mode mismatch in this portion. In addition, it is possible to use an EB process which is able to form the tip end to be narrower, but an increase in manufacturing costs is caused.
On the other hand, J. Zhang, M. Yu, G.-Q. Lo, and D.-L. Kwong, “Silicon-Waveguide-Based Mode Evolution Polarization Rotator,” IEEE Journal of Selected Topics in Quantum Electronics, Vol. 16, Issue 1, pp. 53-60 (2010) is exemplified as an example of realizing the same polarization rotation process as that of L. Chen, C. R. Doerr, and Y.-K. Chen, “Compact polarization rotator on silicon for polarization-diversified circuits,” Optics letters, Vol. 36, Issue 4, pp. 469-471 (2011) described above by only using silicon. However, as described above, when a tapered tip end portion has a width of approximately 100 nm in a connection portion, a loss due to the mode mismatch occurs.
In Junji YAMAUCHI, Masatoru SHIMADA, Tadashi NAKAMURA, Yuu WAKABAYASHI, and Hisamatsu NAKANO, “Reflection Loss of L-shaped and Inclined Waveguide Type Polarization Converter”, Proceedings of The Institute of Electronics, Information and Communication Engineers, 2011, C-3-52, a structure is disclosed in which an L-shaped waveguide is connected to a rectangular waveguide. This is an element in which a waveguide of which a polarization axis is inclined is connected to an original waveguide, and thus the polarization is rotated by using a difference in effective refractive indices between the two modes. However, in this connection portion, a loss occurs, and the effective refractive index depends on the wavelength. Therefore, wavelength dependency occurs.
In addition, in C. Alonso-Ramos, S. Romero-Garcia, A. Ortega-Monux, I. Molina-Fernandez, R. Zhang, H. G. Bach, and M. Schell, “Polarization rotator for InP rib waveguide,” Optics Letters, Vol. 37, Issue 3, pp. 335-337 (2012), a method of connecting a waveguide having an inclined axis by similarly deforming a rib type waveguide is disclosed as an example of using InP. A polarization rotation element with a low loss is realized from the rib type waveguide by designing a length suitable for the L-shaped waveguide in which the TE mode and the TM mode are mixed, and by exciting both of the modes with a low loss.
However, problems occur such as a manufacturing tolerance of a taper portion for exciting both of the modes and the occurrence of wavelength dependency as with Junji YAMAUCHI, Masatoru SHIMADA, Tadashi NAKAMURA, Yuu WAKABAYASHI, and Hisamatsu NAKANO, “Reflection Loss of L-shaped and Inclined Waveguide Type Polarization Converter,” Proceedings of The Institute of Electronics, Information and Communication Engineers, 2011, C-3-52.
As described above, in the related art, a polarization conversion element in which an easy manufacturing process and low wavelength dependency are compatible is required to be realized.